This invention relates to electrical switches generally and, in particular, relates to miniature electrical switches used in electronic circuits comprising components which are mounted by their leads on printed circuit boards.
Modern electronic circuits often provide switch selectable options to perform several functions. Selection of these options is made by factory or field personnel setting or resetting small or miniature switches mounted on circuit boards by their leads. In operation, these switches are much like conventional two-position switches in that in one position they close a circuit and in the other position they open the circuit. In structure, however, the switches are much smaller than conventional switches and thus they require minimum areas on the circuit boards, and adjacent circuit boards on which they are mounted may be closely spaced from one another. The switches include contacts which are designed to carry low level logic currents between logical elements such as transistors and integrated circuits.
The miniature switches usually are set in one position or the other and remain in that position during the operational life of the circuit board or related equipment in which they are installed, but occasionally are actuated to the opposite position to re-select the desired option.
These switches generally are of two types. The first type of switch is essentially a scaled-down wall switch. It is formed in an assembly resembling an integrated circuit and has switch leads arranged in two rows along the bottom of the assembly spaced from one another at the same standard distances designated for integrated circuit leads. This type of switch assembly, also known as a Dual-Inline-Package switch assembly or DIP-switch assembly comprises a plurality of slide- or snap-action contact switches arranged laterally across the assembly with the two leads of each switch being arranged opposite one another. Each switch includes either a button which is reciprocated rectilinearly to actuate the slide contacts or a toggle arm which is reciprocated about a pivot to actuate spring-loaded snap contacts. Typically, such a DIP-switch assembly is designed to have the same dimensions as an integrated circuit package to facilitate printed circuit board layout and increase the circuit board component density. The main advantage of DIP-switches is the high switch density they provide.
There are several problems with such DIP-switch assemblies. The first is that the number of parts necessary to fabricate each switch of the assembly is excessive. Each snap-action switch typically includes a lower body portion, two contact-lead members, a toggle arm, a pivot pin, a detent member, a spring, a ball bearing, a piece of tape to separate the contact members from contaminants and an upper body portion. Optionally, a conductive grease is applied to the contact members and often the whole assembly is potted or molded with a potting compound.
The number of parts varies with different designs, but to assemble one eight switch assembly, approximately 60 individual parts must be handled in an assembly process that does not normally lend itself to mass production techniques. These are small parts which are difficult to handle and which must be precision manufactured.
A second problem with DIP-switch assemblies is the inadvertent setting or resetting of individual switches when the switch assembly is mounted on a printed circuit board. This can occur by action of vibration or by the hand or cuff of a technician or user inadvertently engaging the toggle arm of a snap-action switch and setting or resetting a switch to the opposite position intended. When this occurs, a technician who has been trained to understand the option selection settings is required properly to place the toggle arm in its correct position. This problem is not as prevalent with slide-action switches where the buttons are closer to the switch body than the toggle arms, and further to reduce the possibility of this problem, the buttons are protected by an additional member known as an overcover overlaid on top of the slide-action switch assembly.
A third problem with DIP-switches is that high resistance coatings can form on the switch contacts and result in a high resistance therebetween even when the switch contacts are closed, providing an incorrect logic level in the electronic circuit. These high resistance coatings are formed by contaminants such as airborne pollutants, circuit board manufacturing chemicals and even elimination products from the potting compound joining together the parts of the switch assembly. Logic level currents of typically several milliampers are normally insufficient at logic level voltages to break through these coatings.
An example of such airborne pollutants is the oil thrown into the air by the electro-mechanical equipment in which such DIP-switches are used. This airborne oil coats all of the components of the equipment with a fine, oily film and can migrate between the parts of the DIP-switch assembly to the contacts to form the described coatings. In manufacturing, solvents are used to clean assembled circuit boards and their components. When these solvents evaporate, they can leave solid residues on the contacts which form the described coatings. This problem has been reduced by placing a strip of tape over the switch contacts to reduce the migration of contaminants to the switch contacts, but this has not eliminated the problem.
A fourth problem with DIP-switches is that their applications are limited to switching logic level signals of typically several milliamperes at approximately five volts. They are not recommended for carrying power currents of typically hundreds of milliamperes at approximately 24 volts. This is because the DIP-switch contacts are designed to be small so that they may be accommodated in an assembly having the same dimensions as an integrated circuit. This results in the contact area being small and the current density being high. At power current levels, the high current density can result in the contacts burning out. The problem of contact burnout is avoided by not using DIP-switches to carry or switch power currents.
A fifth problem with DIP-switches is that they are commercially available only in assemblies of standard numbers of switches, such as four, eight, ten or twelve. A DIP-switch assembly of such as nine switches is not commercially available except upon special order and substantially increased price. Thus, a manufacturer having a circuit requiring a DIP-switch assembly of some number of switches other than standard must select an available assembly having a greater number of switches than is required and not use the additional switch or switches, which is wasteful. This waste becomes significant when large numbers of switch assemblies are used.
These problems, generally, have not been eliminated in the highly developed DIP-switches currently available, but simply are tolerated. The development of these DIP-switches, which has occurred over a long period and which recently has stagnated, has not addressed the fundamental reasons for these problems but has only reduced the severity of the problems, with attendant increase in the cost and complexity of such switches. Essentially, the present DIP-switches are highly developed, but scaled down, wall switches. But because of the high switch component density which they provide, DIP-switches are used in large numbers over other types of switches.
The second type of switch is a screw-type of switch and is well-known. A perforation is made through the printed circuit board and a front conductor is printed about the perforation on the front side of the board while a rear conductor is printed about the perforation on the rear side of the board. A nut is swaged and soldered in place on the rear side of the board in contact with the rear conductor, with the threaded opening of the nut axially centered with the perforation. A screw then is threaded through the perforation and into the nut so that its head is engageable against the front conductor. The screw-switch thus formed between the front and rear conductors may be closed by screwing home the screw into the nut to engage the head against the front conductor and may be opened by backing off the screw from the nut to disengage the head from the front conductor.
Screw-switches also present problems. The first is that they require too much area on the printed circuit board. The nuts and screws typically are large to expedite the mechanical operations of swaging the nut on the rear of the board and threading the screw into the nut. The designs of different screw-switches require different areas, but in one design, one screw-switch requires approximately the same area as an assembly of four DIP-switches. Component density must be maintained high on circuit boards to realize cost savings, and the more area required for switches lowers the component density and increases the cost.
A second problem is maintaining the screw head disengaged from the front conductor but threaded in the nut. Simple vibration can rotate the screw while loosened so that either the head engages the front conductor to close the switch, or the screw becomes unthreaded from the nut and falls from the board into other electronic circuitry. Either event is undesirable. Attempts to solve this problem include designing a special screw with an unthreaded, smaller diameter shank portion in the shaft so that the screw must be positively threaded past the shank portion for either tightening the screw into the nut or removing the screw from the nut and board. Another solution provides a removable insulator mounted on the screw shaft to separate the front conductor and screw head and maintain the switch open even when the screw is tightened into the nut. A third solution provides means in the nut which hold the screw in any rotational position under vibrational force, but which provides for rotation of the screw under a greater, intentionally applied force. All of these attempted solutions require specially designed parts.
A third problem is the manual labor required to assemble a nut and screw screw-switch on a board. The swaging of the nut on the board and the threading of the screw into the nut are normally manual operations which are not readily automated. One attempted solution is to provide an assembly providing a single screw-switch assembly which may be mounted on a board by its two depending leads. This solution simplifies the assembly of new boards laid-out to accommodate such an assembly, but this assembly cannot be retro-fitted into existing boards having a screw-switch assembly. Further, this single screw-switch assembly requires as much board area as a conventional screw-switch.
For these reasons, screw-switches have been used in small numbers and then mainly in single station applications where, for example, the switch contacts carry a power level current to such as the coil of a solenoid. Thus, while DIP-switches and screw-switches are used for similar and often the same applications, DIP-switches have been used much more extensively because of the higher switch density they provide.
What is desired is a switch assembly which may be retrofitted into the DIP-switch assembly layouts of existing boards and which maintains the switch density which a DIP-switch assembly presently provides, but which avoids or eliminates the problems accompanying DIP-switches.